In the genus Euplectes (17 species of widowbirds and bishops), red carotenoid-based coloration has been found to function in male contest competition over territories and appears to exploit a generalized receiver bias by which redder (more longwave) hues are perceived as more aversive or intimidating. A major piece missing, however, is that among the 5 most extensively red species, forming the clade of “red bishops,” the agonistic function has remained unexplored or undetected. Moreover, does the receiver bias remain generalized (“open-ended”) after coevolution of the most exaggerated signal phenotypes? In this field experiment with southern red bishops E. orix, we increased the color hue 27nm, beyond its natural range, and compared the success in territorial competition of these supernormal red males to that of control-red and green (down-manipulated) males. The treatment, but no pre-manipulation colorimetrics or morphometrics, had a significant effect on territory establishment; 76% of super-red males, 45% of control-red males, and 17% of green males were recorded as territory holders at the end of the experiment. The receiver bias, that is, a generalized aversion of redder hues, thus appears to remain after the signal elaboration process, likely (but not necessarily) reinforced by adaptive receiver coevolution, through, for example, exploitation resistance or signal honesty.